Pleiotropism and Examples

A single gene may have two or more phenotypic expressions. The multiple
phenotypic effect of a single gene is called pleiotropism.

Examples of Pleiotropism

I. The allele ‘Ay’ in mice has two phenotypic effects

Controlling lethality

controlling yellow coat colour.

II. In human beings, Marfan’s syndrome, characterized by long
limbs, slender body, hypermobility of joints, lens dislocation and
susceptibility to cardiac diseases are caused by single pleiotropic gene.

III. Sickle cell disease

It is a genetically transmitted haemolytic
disease.

Homozygotes have two copies of abnormal gene (beta
haemoglobin) leading to formation crescent shaped or sickle shaped hemoglobin
(Hbs). These cells have very low oxygen carrying capacity. Thus impair
circulation. This often led to renal failure, cardiac failure and thrombosis.
Thus a single mutation in a gene has multiple consequences in numerous tissues.
That is, a single gen has multiple effects on different sites or tissues
referred as pleiotropism.

But in heterozygotes (having one abnormal copy and one normal
copy), there is an advantage called heterozygote advantage. These heterozygotes
are protected against most lethal form of malaria.